1. Field of the Invention
This invention relates to signal processing. More specifically, this invention relates to signal processing as applied to wireless communications.
2. Description of Related Art and General Background
Spread spectrum communication techniques offer robustness to noise, low transmission power, and a low probability of intercept. For such reasons, much of the early development of spread spectrum technology was performed by military researchers. Recently, however, the advantages of this technology have led to its increasing use for consumer applications as well: most notably, in advanced digital cellular telephone systems.
Communication systems that support multiple individual signals over a single channel must employ some technique to make the various signals distinguishable at the receiver. In time-division multiple-access (TDMA) systems, the individual signals are time-compressed and transmitted in nonoverlapping intervals such that they are orthogonal (and thus separable) in time space. In frequency-division multiple-access (FDMA) systems, the signals are bandlimited and transmitted in nonoverlapping subchannels such that they are orthogonal in frequency space. In code-division multiple-access (CDMA) systems, the signals are spread through modulation by orthogonal code sequences such that they are orthogonal in code space and may be transmitted across the same channel at the same time while remaining distinguishable from each other at the receiver.
Whereas most other communication techniques modulate a carrier signal with one or more data signals alone, spread spectrum techniques also modulate the carrier with a pseudorandom noise or xe2x80x98pseudonoisexe2x80x99 (PN) signal. These PN signals are selected to have minimal cross-correlation, and their properties and generation are discussed in more detail in, e.g., Modern Communication Systems: Principles and Applications, Leon W. Couch III, Prentice Hall, 1995, pp. 381-83, and chapter 2 of CDMA: Principles of Spread Spectrum Communication, Andrew J. Viterbi, Addison-Wesley, 1995. In the frequency-hopping variant of spread spectrum systems, the value of the PN signal at a particular instant determines the frequency of the transmitted signal, and thus the spectrum of the signal is spread. In the direct sequence spread spectrum (DSSS) variant, the bit rate of the PN signal (called the xe2x80x98chip ratexe2x80x99) is chosen to be higher than the bit rate of the information signal, such that when the carrier is modulated by both signals, its spectrum is spread.
In a CDMA DSSS system, then, each individual signal is modulated by a data signal and a pseudonoise (PN) signal of predetermined period that is at least nearly orthogonal to the PN signals assigned to all other users, thus spreading the spectrum of the signal while rendering it distinguishable from the other users"" signals. Before spreading and modulation onto the carrier, the data signal typically undergoes various encoding and interleaving operations designed, for example, to increase data redundancy and allow error correction at the receiver. The data signals may also be encrypted to provide extra security against eavesdroppers. The generation of CDMA signals in a spread spectrum communications system is disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d and assigned to the assignee of the present invention.
Various schemes exist for modulating baseband data signals onto RF carriers. These schemes typically operate by varying the amplitude phase, and/or frequency of one or both of the in-phase (I) and quadrature (Q) components of the carrier according to the data symbol to be transmitted at any particular instant. For example, CDMA DSSS systems commonly use a variant of phase-shift keying (PSK), in which the phase states in the carrier components correspond to data symbols being transferred. Phase-shift keying modulation may also be used in many non-CDMA and non-DSSS applications as well.
In one example of a system using binary PSK (BPSK) modulation, a transition of the carrier from a base phase state (defining a phase of zero) to a second phase state which is different by 180 degrees (i.e. a phase shift of xcfx80 radians away from zero) may be designated to indicate a transition from a data symbol 0 to a data symbol 1. The converse phase shift of xcfx80 radians back to zero would then be designated to indicate a transition from a data symbol 1 to a data symbol 0. Between these transitions, the phase of the carrier indicates whether a data symbol 0 is being transmitted (phase of zero) or a data symbol 1 instead (phase of xcfx80 radians).
An improved ratio of data rate to bandwidth may be obtained by using quadrature PSK (QPSK) modulation, in which the data symbols are encoded into 180-degree shifts in both the I and Q components. This scheme results in a maximum carrier phase shift of 180 degrees at every symbol transition. A variant of QPSK called offset QPSK (OQPSK) staggers the symbol transitions across the I and Q components in time, thereby reducing the maximum instantaneous phase shift in the carrier to 90 degrees. The above-mentioned and other variants of PSK modulation are well known in the art.
An apparatus is described which receives a complex signal having a phase and outputs another complex signal having a phase. Each of these complex signals has a first and a second component. The first component of the output signal is a difference of the components of the input signal, the second component of the output signal is a sum of the components of the input signal, and a phase angle of the output signal is rotated as compared to a phase angle of the input signal.